The present invention relates to an improvement of an apparatus which irradiates light on living tissue of a patient, and detects light transmitted through or reflected by the living tissue, thereby determining concentrations of light absorbing substances in blood.
Pulsating components of light transmitted through or reflected by living tissue include information on light absorption characteristics of light absorbing substances in blood.
Accordingly, by measuring pulsating components of light transmitted through or reflected by living tissue with use of a plurality of wavelengths, ratios between concentrations of a plurality of light absorbing substances in blood can be measured.
This basic principle has a wide range of applications, and is given the generic name of pulse photometry. One application of the principle is a pulse oximeter which determines arterial oxygen saturation continuously in a noninvasive manner.
Since a change in the condition of a patient requires an immediate response, an important consideration for such a pulse oximeter is use in a continuous monitoring mode. Also, such a pulse oximeter is desirably portable so as to be carried from one room to another without requiring insertion in a power receptacle. Power consumption is a problem for such a portable type pulse oximeter, and reduction in (conservation of) power consumption has been desired.
The same problem described above applies to other apparatus for determining concentrations of light absorbing substances in blood, such as hemoglobin concentration, dye concentration, bilirubin concentration, and blood glucose level.
A configuration of a conventional pulse oximeter and light-emitting timings of LEDs serving as a light source will be described by reference to FIGS. 5A and 5B. Such a configuration is disclosed in “Clinical Engineering Vol. 7, No. 2 (1996) pp 102-110” and “Handbook of Medical and Biological Engineering Equipments (revised version; 1996), pp 130-132”, for example.
As shown in FIG. 5A, a pulse oximeter comprises a probe P attached to a portion (e.g., a finger) of a patient, and a main body H for processing a signal output from the probe.
The probe includes an LED section 1 serving as a light-emitter which comprises a first LED 1a for emitting red light of a first wavelength and a second LED 1b for emitting infrared light of a second wavelength; and a light-receiver constituted of a photodiode 3 for receiving light transmitted through or reflected by tissue of the patient to whose finger, or the like, the probe is attached.
The first wavelength of red light is in the vicinity of 680 nm and the second wavelength of infrared radiation is in the vicinity of 940 nm are generally used, and the LEDs 1a, 1b in the LED section 1 are alternately caused to illuminate.
The main body H of the pulse oximeter includes a current-voltage converter 4, a demodulator 5, a pulsation ratio detector 7, an attenuation ratio calculator 8, an oxygen saturation calculator 9, an LED driving current calculator 10, and an LED driver 2.
In the configuration shown in FIG. 5A, intensity of light, which has been emitted from the LED section 1, passed through tissue such as a finger, and reached the photodiode 3, is converted into current by the photodiode 3.
The intensity of light which has been converted into current is converted into voltage by the current-voltage converter 4 in the main body H, and separated into a transmitted-light signal of red light and that of infrared light, respectively, by way of the demodulator 5.
Each of the thus-separated transmitted-light signals indicates a waveform in which a pulse wave component (AC component) is superimposed on a DC component (DC component).
The respective transmitted-light signals are separated into the DC component and the AC component at the pulsation ratio detector 7, whereby a pulsation ratio (AC component/DC component) is calculated.
For calculation of oxygen saturation, an attenuation change ΔA must be obtained, which can be approximated by a pulsation ratio. Accordingly, pulsation ratios of the respective red light and infrared light are extracted as attenuation changes (ΔA1, ΔA2) from the pulsation ratio detector 7. The thus-extracted pulsation ratios are processed by the attenuation ratio calculator 8 to calculate a ratio thereof (Φ=ΔA1/ΔA2), and converted into oxygen saturation in the oxygen saturation calculator 9. A pulse rate can also be obtained simultaneously.
Next, light-emitting timings of the wavelengths 1 and 2 emitted alternately from the first LED 1a and the second LED 1b will be described by reference to FIG. 5B.
In FIG. 5B, “a” indicates a light-emitting timing of the first LED 1a and “b” indicates the same of the second LED 1b; and “a” and “b” are controlled by the LED driver 2 so as to alternately illuminate in a frequency range of hundreds of Hz to some KHz. “c” is an output from the photodiode 3 in response to received light, and indicates a value of intensity of transmitted light of the first wavelength and that of the second wavelength which have been converted into electric signals. “a1” indicates intensity of the transmitted light of the first wavelength received by the photodiode 3, and “a2” indicates the same of the second wavelength.
In a conventional control for reducing power consumption, attention has been focused on the DC component, and drive current of the LED section 1 has been controlled such that the LED driving current calculator 10 calculates an optimum LED driving current in accordance with the DC component of the transmitted light, whereby the optimum LED driving current is supplied to the LED section 1 by the LED driver 2.
Control modes of drive current include decreasing amplitude of a current pulse, or narrowing a pulse width of a current pulse. Examples of such control modes are shown in FIG. 3. The upper diagram shows an example where a width of a current pulse in a single period is narrowed, and the lower diagram shows an example where an amplitude of a current pulse is decreased.
In the conventional pulse oximeter, in order to control the LED driving current for the purpose of power-saving, attention is focused on the DC component. When the DC component is large, an amount of transmitted light received by the photodiode 3 is determined to be large, and the LED driving current is reduced so as to reduce an amount of light emitted from the LED section 1.
However, in a case of a patient whose pulse wave is small, the AC component is small even when the DC component is large. Accordingly, when the amount of light emitted from the LED section 1 is reduced, detection of the AC component which can maintain measurement accuracy as a pulse oximeter becomes difficult.
Therefore, when the LED driving current is controlled by reference solely to a value of the DC component, measurement of an arterial oxygen saturation (SpO2) can be performed in some cases but not in other cases in accordance with the status (magnitude of an AC component) of a patient.
In order to avoid the problem, there must be supplied an LED driving current that consistently maintains an amount of light emitted from the LED, by which the oxygen saturation (SpO2) of a patient who is assumed to have a small pulse wave (i.e., AC component thereof is small) can be measured.
In this case, there arises a problem that, since an LED current larger than required is supplied to a patient having a large AC component, sufficient control for power-saving cannot be achieved.
This situation will be described by reference to FIG. 4, which is a graph where the horizontal axis represents a DC component of transmitted light to be input into the current-voltage converter 4, and the vertical axis represents a pulsation ratio, which is a ratio between the AC component and the DC component of the transmitted light. The thick solid line indicates a boundary at which the AC component is 100 (pA), (this value is set to a magnitude with which a sufficient signal to noise ratio can be ensured in relation to noise of the measurement system, and varies in some cases depending on the type of a pulse oximeter; and the value is not limited to 100 (pA)) which is a limit value where an arterial oxygen saturation (SpO2) can be measured by a pulse oximeter.
A pulse oximeter has a characteristic such that when the AC component falls lower than 100 (pA), measurement of the AC component becomes impossible. More specifically, the right side including the boundary indicates a region where the arterial oxygen saturation (SpO2) can be calculated, whereas the left side indicates a region where the arterial oxygen saturation (SpO2) cannot be measured.
Here, on the assumption that a DC component of transmitted light obtained is 100 (nA) and a pulsation ratio is 1 (%) when an electric current of Ao (mA) is supplied to the LED section 1, the pulse wave comes to a point “a” in FIG. 4.
Next, while attention is focused on the DC component, when the LED driving current section 1 is caused to decrease to an appropriate value A (mA) so as to decrease the DC component from the present 100 (nA) to 10 (nA). At this time, on the assumption that the pulsation ratio does not change during a short period of time, the point “a” moves horizontally to the point “b” in parallel with the axis. In this case, as understood from FIG. 4, measurement of the arterial oxygen saturation (SpO2) remains to be possible even when the LED current is decreased from Ao (mA) to A (mA).
However, under such a control where attention is focused on the DC component, even on the assumption that an AC component of light transmitted through a patient is always small and, upon supply of electric current of Ao (mA) to the LED section 1, a DC component of 100 (nA) is obtained, when the AC component is assumed to be 0.5 (nA), the pulsation ratio becomes 0.5 (%), which is plotted as “c” in FIG. 4.
In this case, when the value thereof is caused to decrease from the present 100 (nA) to 10 (nA) with attention focused on the DC component, crossing the boundary occurs at point “d,” which indicates falling into the region where measurement of the arterial oxygen saturation is impossible at 10 (nA).
As described above, when control is executed with attention focused only on the DC component, there are some cases where measurement of the arterial oxygen saturation is impossible even though the DC component is the same 100 (nA) when the DC component is caused to decrease to 10 (nA), depending on a status of a patient.
In order to avoid the problem, the LED driving current must be supplied such that the DC component is consistently 1,000 (nA) or higher so as to enable detection of the pulse wave at all times independent of the status of a patient (i.e., the AC component).
As mentioned above, the control that focuses attention solely on the DC component enables reduction of the LED driving current from the point “a” to the point “b.” In spite of this fact, an LED driving current of 1,000 (nA) or higher is achieved at all times. Hence, difficulty is encountered in substantially saving power.